The results of an investigation into a fatality have highlighted potential hazards that may exist in plates that had been indented during operations or in other cases. Plates on equipment, other than the excavator bucket involved in this incident, can pose similar hazards. Indentation is a major contributing factor to elastic spring back potential. Other factors may also play a role. While the fatality occurred with a quench and tempered steel wear plate, similar hazards might exist with other metallic and non-metallic materials (e.g. stainless steel, fibreglass, mild steel, aluminium or plastic). This presentation discusses the incident, the contributing factors and ways to help prevent similar occurrences.

1. Elastic springback in plates
Theo Kahl
Inspector of Mines

2. Background
A boilermaker was using an air carbon arc gouging process to
remove an external wear plate from an excavator bucket. The
plate unexpectedly sprang up approximately 1200 mm. About
midway in this upward movement, it struck the boilermaker on his
forehead, fatally injuring him.

3. The bucket

4. Cracks
Without long, through-thickness cracks, the
wear plate would probably have released in
a controlled manner.

5. Investigation
Core Question
Why was there so much elastic springback in the wear plate?

6. Causes of elastic springback
1. Misshape

7. Causes of elastic springback
2. Welding
• Directly after welding, the
molten metal solidifies and
then contracts as it cools.
This contracting weld
material causes stress and
strain both within the plate
and welds.
• The amount of residual
stress from welding is
unlikely to cause the amount
of elastic springback that
was encountered. It did
however add to the overall
stress prior to cracking.

8. Causes of elastic springback
3. Wear
• When a plate is bent or rolled, it is plastically deformed. One surface is under compressive residual stress, while the
opposite is under tension. When it is newly rolled, the opposing residual stresses are in balance and maintain the
desired shape of the bent plate.
• During operation, one surface wears away, disturbing the balance and contributing to elastic springback potential.

9. Causes of elastic springback
4. Indentation

10. Causes of elastic springback
4. Indentation (Continued)
If a plate is impacted or permanently indented during operations, the strain and stress profiles are likely to
be affected. There are different configurations:
• For a profiled (e.g. rolled) plate, if the magnitude of indentation is sufficient, the material yields in the
opposite direction, effectively cancelling or reducing the residual stresses needed to maintain the plate
profile. This contributes to elastic springback potential.
• Where the impact is severe enough to cause yielding, the plate permanently elongates (stretches) on
the opposite side to the impact. The elongated material tends to lift up the plate and contribute to elastic
springback.
• Multiple, overlapping indentations may have had a cumulative effect, depending on their relative
positioning.
• A plate can be indented even if it is closely supported by a hard and strong backing material. An
unsupported or weakly supported plate will however indent more than a supported one for the same
impact force. (See slide about ‘dirt’)
For the plate involved in this incident, indentation was the primary contributor to elastic springback.

11. Causes of elastic springback
5. Poisson expansion [pwä-ˈsōⁿ]
• As a plate is being indented
during operations, it
compresses in the through-
thickness direction. This causes
the material to expand in all
directions within the plate plane;
a phenomenon called Poisson
expansion.
• Stresses from Poisson
expansion may add to
springback potential.

12. Causes of elastic springback
6. Dirt ingress
• Static pressure
• Soft backing increases level of
deformation

13. Springback from ‘misshape’ vs ‘indentation’
• Indentations increase the second moment of
area of the plate cross-section
• If the plate is considered to be a ‘cantilever
beam’ it can carry a larger load than a non-
indented plate.
• Similarly, an indented plate will have more
elastic springback than one that is not indented.

14. Measuring
tapes
• Concave cross-section
(~ “Indented plate”)
• Flat cross-section
(~ “Non-indented plate”)

15. Motion of
wear plate
Time to midpoint:
50 ms
Average human
reaction time to
visual stimulus:
250 ms

16. Animation of plate movement

17. Other materials
• This incident: Q&T steel plate
• Other materials:
• Metals
• Stainless steel (e.g. bin liners)
• Mild steel
• Aluminium
• Copper
• Etc
• Non-metals
• Fibre glass
• Plastic
• Ceramics
• Etc

18. Other incidents
• In two similar but separate
incidents, workers were struck
while removing indented sections
near the rim of rear dump truck
trays.
• Violent elastic springback was
observed during the removal of an
under-tub wear plate from a
dragline. The plate had probably
been indented when the dragline
was walked over an uneven
surface.
• A worker was hit while removing a
wear liner plate (push pad) from a
dozer blade.

19. Recommendations
• Awareness
• Detection
• Dents
• Deformation from dirt
• Cracks
• Wear
• Forceful installation
• Q&T steels
• Large plates

20. Recommendations (Continued)
Better design
For multiple small
wear plates there
is less stored
energy per plate

21. Recommendations (Continued)
Controlled release
• When an installed plate is being removed, measures should be in
place to release it in a controlled and systematic manner. This
applies to both gouging and unbolting.
• Before the first fastening element (welding, bolting or other) is
removed, assess whether the remaining elements will have
enough strength to contain potential springback energy.
• If there is doubt about the integrity of the original fastening
elements, extra welding or replacement of the original bolts
should be considered.

22. Recommendations (Continued)
SHMS
Work within a good SHMS that includes (but is not limited to)
elements such as risk assessment, change management, training,
supervision and a system to keep and disseminate safety
information about similar earlier jobs.

23. Thanks for your attention!